This invention relates to the field of optics and lasers, and more specifically to a method and apparatus of integrating one or more optical waveguides on a glass substrate and of forming lasers therein.
The telecommunications industry commonly uses optical fibers to transmit large amounts of data in a short time. One common light source for optical-fiber communications systems is a laser formed using erbium-doped glass. One such system uses erbium-doped glass fibers to form a laser that emits at a wavelength of about 1.536 micrometer and is pumped by an infra-red source operating at a wavelength of about 0.98 micrometer. One method usable for forming waveguides in a glass substrate is described in U.S. Pat. No. 5,080,503 issued Jan. 14, 1992 to Najafi et al., which is hereby incorporated by reference. A phosphate glass useful for lasers is described in U.S. Pat. No. 5,334,559 issued Aug. 2, 1994 to Hayden, which is hereby incorporated by reference. An integrated optic laser is described in U.S. Pat. No. 5,491,708 issued Feb. 13, 1996 to Malone et al., which is hereby incorporated by reference.
There is a need in the art for an integrated optical system, including one or more high-powered lasers along with routing and other components, that can be inexpensively mass-produced. The system should be highly reproducible, accurate, and stable.
The invention provides, among its embodiments, a system and method for forming an optical system including a high-powered laser on a glass substrate, and a resulting glass-substrate-based optical system and method for operating the optical system. The invention is further directed to methods for forming optical waveguides on glass substrates, including: forming substrates with multiple waveguides and including wherein at least two of the multiple waveguides have differing wavelengths.
Further embodiments of the invention provide a laser component that includes a glass substrate doped with a laser species and having one or more, preferably multiple, waveguides defined by channels within the substrate. (As used herein, a xe2x80x9cchannel within the substratexe2x80x9d is meant to broadly include any channel formed on or in the substrate, whether or not covered by another structure or layer of substrate.) Each substrate waveguide (or xe2x80x9cchannelxe2x80x9d) is defined within the substrate as a region of increased index of refraction relative to the substrate. The glass substrate is doped with a laser species which can be optically pumped (preferably a rare-earth element such as Er, Yb, Nd, Ho, Tm, Sm, Tb, Dy or Pr or a combination of such elements such as Er and Yb) to form a laser medium which is capable of lasing at a plurality of frequencies. Mirrors or distributed Bragg reflection gratings may be located along the length of a waveguide for providing feedback to create a laser-resonator cavity. One or more of the mirrors or reflection gratings is made partially reflective for providing laser output.
The laser component may constitute a monolithic array of individual waveguides in which the waveguides of the array form laser resonator cavities with differing resonance characteristics (e.g., resonating at differing wavelengths). The component may thus be used as part of a laser system that outputs laser light at a plurality of selected wavelengths. In certain embodiments of the invention, the resonance characteristics of a waveguide cavity are varied by adjusting the width of the channel formed in the substrate which thereby changes the effective refractive index of the waveguide. The effective refractive index can be changed by modifying the diffusion conditions under which the waveguides are formed as described below. Changing the effective refractive index thus changes the effective length of the waveguide cavity which determines the wavelengths of the longitudinal modes supported by the cavity. In another embodiment, the resonance characteristics of the waveguide cavities are individually selected by varying the pitch of the reflection gratings used to define the cavities which, along with the effective refractive index for the propagated optical mode, determines the wavelengths of light reflected by the gratings. In still other embodiments, the location of the gratings on the waveguides is varied in order to select a laser-resonator cavity length that supports the desired wavelength of light.
In a preferred embodiment, the waveguide or multiple waveguides, optionally as part of a laser element, are constructed from a glass substrate which is a phosphate alkali glass doped with a rare-earth element such as Er or Yb/Er. In the case of Yb/Er doped glass, it is preferred for maximal lasing efficiency that the Yb/Er ratio is from approximately 1:1 to 8:1, particularly 3:1 to 8:1. This has been discovered as a result of investigation as to the optimal erbium and ytterbium rare earth ion concentrations in rare earth doped laser glasses, in particular as erbium and ytterbium 1.54 xcexcm laser sources and amplifiers employed in the fields of telecommunication and data transmission. Prior to this investigation, erbium/ytterbium doped glasses for such applications were typically characterized by low erbium concentration (generally much less than 1 wt % Er2O3 content) with a corresponding high ytterbium content (typically at a ion ratio of greater than 10 ytterbium ions for each erbium ion input to the glass); such glasses are disclosed in, for example, U.S. Pat. No. 5,334,559 and U.S. Pat. No. 5,491,708. Such high levels of Yb were initially expected to yield high output powers and high slope efficiency based on prior experience with silicate and phosphate glass formulations, see for example U.S. Pat. No. 4,962,067. The prior art taught that low erbium doping levels (at most 0.15 mole % Er2O3) were required to avoid self quenching effects, and that the ytterbium content be set as high as possible (basically input Yb2O3 to near the solubility limit in the glass, at least 6 mole % Yb2O3) in order to optimize the amount of pumping light absorbed within the laser glass.
However, according to this invention, it has been discovered that glasses with a Yb/Er rare earth content of from about 1 :1 to 8:1 (particularly the glass called NIST-1T or IOG-1, a sodium-aluminum-phosphate glass having a content in melt of 1.15 wt % Er2O3 and 4.73 wt % Yb2O3) demonstrated a higher output power (up to 180 mW compared to a prior high of only 16 mW) and higher slope efficiency (of at least 28% compared to a prior high of only 27%).
The inventors first expected that the poor performance of the prior art glasses was attributed to, at least in part, residual hydroxyl groups left in the glass. Rare earth ions excited in the glass are known to exchange energy with hydroxyl vibrational overtones in the glass, effectively robbing the excited state ions of the stored energy otherwise used to produce amplified laser emission. To investigate this possibility a series of sodium-aluminum-phosphate glasses containing 0.5wt % Er2O3 and 8.94wt % Yb2O3 with different hydroxyl content were prepared. The residual hydroxyl content in these glasses is proportional to the measured infrared absorption at 3.0 um and is detailed in Table 1.
The results of this work indicated that although residual hydroxyl content certainly was detrimental to laser performance, it could not alone account for the poor performance of devices fabricated in these three glasses. In fact, melt NIST-1H and NIST-1L were characterized by an absorption at 3.0 um below 2.0 cmxe2x88x921, a level expected to be low enough not to significantly influence laser performance. In particular, the absorption level in melt NIST-1L was less than 1.0 cmxe2x88x921, a threshold value below which the prior art has shown is not an issue in evaluating laser performance of a given laser glass, see Cook, L. M. et al, Proc SPIE Vol. 505, pp 102-111 (1984).
As a result, it was determined that the selected doping levels of Er and Yb themselves must be far from optimal. In particular, it was realized that there are performance tradeoffs related to the Yb-doping concentration, the Er-doping concentration, and the Yb/Er-doping ratio within the glass. For example, as indicated earlier, it had been generally thought that the more Yb doping will always result in better laser performance. The inventors instead selected an alternate approach to optimizing the amount of Er and the Yb/Er ratio. The approach involves evaluating the relative performance tradeoffs between Yb-Er cross-relaxation efficiency and the total number of inverted Er ions in the laser cavity.
Spectroscopic evaluations of the NIST-1 glass were performed to determine the cross relaxation coefficient of the Yb-Er energy transfer mechanism. The cross relaxation efficiency, xcex7, of ytterbium to erbium ion in the glass was estimated by the inventors to be given by,
xcex7=Ixe2x88x92xcfx84Ybxe2x88x92Er/xcfx84Yb
where xcfx84Ybxe2x88x92Eris the measured lifetime of the Yb3+2F3/2 level in a codoped sample with Er (measured at 1.79xc3x9710xe2x88x923  seconds) and xcfx84Yb is the measured lifetime of Yb3+2F5/2 level in a sample with no erbium (measured at 1.37xc3x9710xe2x88x923 seconds). The value of xcex7 was thus calculated to be 0.87. Additional description of this modeling method, but applied to silicate glasses having greatly inferior laser properties, is provided in xe2x80x9cRigorous Scalar Modeling of Er and Er/Yb-doped Waveguide Lasersxe2x80x9d (Veasey, Gary, Amin) which is incorporated by reference.
The doping selection modeling method provided the following conclusions: as more Yb is incorporated into a glass with a constant Er concentration, the average distance between Er ions and Yb ions decreases, thus causing a rise in the cross relaxation efficiency of Yb-Er energy transfer. The disadvantage of this is that more of the pump power is absorbed in a shorter distance. The result is that less Er ions will be inverted in a certain length, thus there will be less gain available as well as additional reabsorption loss in the 3-level Er ion laser system. Laser performance will suffer. The penalty will be higher thresholds and lower slope efficiencies. To obtain the best possible performance, both the Yb-Er cross relaxation and the total number of inverted ions as a function of pump power must be optimized simultaneously. For a particular device, based on the absorption characteristics of ytterbium at the excitation (pump) wavelength (the absorption cross section at the optimal pump wavelength of 980 nm is 14.5xc3x9710xe2x88x9221 cm2) and the intended device length (2.2cm) the modeling pointed to a glass doped with 1.15 wt % Er2O3 and 4.73 wt % Yb2O3 with an Er concentration of 1xc3x971020 ions/cm3. Although Er concentrations up to 6xc3x971020 ions/cm3 can be used, the total doping concentration is constrained only by the solubility for rare earths within the glass host, the levels as high as 10xc3x971020 rare earth ions per cm3 are known, see U.S. Pat. 5,077,240 and 5,039,631. These unusual doping levels were selected despite the conventional wisdom that taught that such an erbium doping content was twice the maximum and that the selected ytterbium content was xc2xcth to ⅕th the prior art levels for a high performance laser device. Based on the results achieved with the discovered optimal doping levels, lasers based thereon can provide more power than currently available from high end diode lasers for telecommunication and data transmission applications. Conventional diode lasers typically have output powers of 2 to 20 mW. The lasers of this invention have output power that is limited only by the available pump power, up to values that can exceed 20 mW, e.g., 25-180 mW. Such power levels allow the use of glass waveguide lasers in metropolitan area networks without the need for expensive optical amplifiers in the network. The advantages are shown particularly for lasers having a length of 15 mm or less.
In preferred embodiments, the doping level, i.e., concentration, of Er ions is from  greater than 0 up to 5 or 6xc3x971020 ions/cm3 and the Er/Yb ratio is about 4:1. Although ratio""s of about 1, 1.5, 2, 2.5, 3, 3.5, 4.5, 5, 5.5, 6, 6.5, 7, 7.5 or 8 to 1 can be useful.
In another preferred embodiment, improved ion exchange methods are used to create channels defining the waveguides in the glass substrate. Generally, a surface of the glass substrate is exposed to an ion-exchange solvent through a mask layer having one or more line apertures corresponding to the channel or channels (for multiple waveguide embodiments).
Processing steps include the generation of waveguides within the glass substrate using K+xe2x88x92Na+ ion exchange through channel apertures fabricated by standard microlithographic techniques. For example, a KNO3 solvent can be used applied through an aluminum mask layer to perform the K+xe2x88x92Na+ ion exchange. The exchange of K+ for Na+ ions in the substrate produces a channel in the exposed portion of higher refractive index than the remainder of the substrate, thus, defining a waveguide. Additionally, nitrate melts of of any mixture of the following nitrate salts: lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, cesium nitrate, copper nitrate, silver nitrate and thallium nitrate, for example, can be used for the ion-exchange to provide the waveguide.
However, the inventors have discovered that the glass is etched significantly when exchange is done in an aluminum crucible in the open atmosphere of box oven. Therefore, according to the invention, it was discovered that performing the ion-exchange in a borosilicate, e.g., pyrex, crucible, as opposed to an aluminum crucible, greatly reduced the amount of surface attack and etching of the phosphate glasses caused by the KNO3 ion-exchange melt. Further, improved control of the water content of the melt by baking the KNO3 at a temperature of 120xc2x0 C. or more, preferably about 120xc2x0 C., for a period of 24-48 hours in an inert argon atmosphere was achieved. Additionally, providing a SiO2 buffer layer coating by sputtering or CVD deposition will reduce surface scattering losses.
The ion-exchange process results in a slightly buried waveguide with the highest index in the waveguide occurring slightly below the surface. This provides the additional advantage of reducing scattering loss. This advantage can be even further enhanced to provide a channel with a shape and size optimized for laser operation by conducting a field assisted ion-exchange process using an electric field, particularly an electric field varied as a function of time wgere the electric field is applied using a liquid sodium nitrate electrode or a solid silver electrode. The temperature can also be varied as a function of time for further tailoring of the index profile within the waveguide.
According to this embodiment, lasers prepared from ion-exchanged waveguides can achieve a high power of over 20 mW, particularly 25 to 180 mW and a high slope efficiency of 28% or higher (previous state-of-the-art in phosphate glass was 16 mW power and 27% slope efficiency by D. Barbier, et al., at Conference on Optical amplifiers and applications, 1995). Further, a high coupling efficiency of pump light from the optical fiber is enabled by the low index change caused by potassium-sodium ion exchange. High coupling efficiency of signal light to the optical fiber is also achieved due to the low index change caused by potassium-sodium ion exchange.
Glasses used as substrates for the above ion-exchange methods include sodium-aluminum-phosphate formulations such as the composition described above, additional glasses are disclosed in, for example, U.S. Pat. No. 5,334,559 and U.S. Pat. No. 5,491,708. Particular applicability is found with glass formulations identified above in the category of rare earth doped sodium-aluminum-phosphate compositions, in particular having the Er/Yb ratio described above. Rare earth ions of choice include erbium and ytterbium, resulting in compatibility with currently commercially available, inexpensive, semiconductor pump lasers and emission within the telecommunications wavelength region about 1.5 xcexcm, particularly 1.54 xcexcm, ytterbium alone to result in laser sources at 1.0 xcexcm, for use in standardized clock apparatus, and neodymium to result in preparing laser sources for small, localized optical networks at 1.06 xcexcm and optical sensors and for gas detection. However, further examples of useful devices for the described ion-exchange process include glasses doped with other rare earth ions, for example, Pr lasers, Ho lasers, Tm lasers, as well as Dy, Sm, Eu and Tb lasers. Representative wavelengths are listed below in Table 4, see, also, Emmett, et al., Physics of Laser Fusion, Vol. IV, xe2x80x9cThe future development of high-power solid state laser systemsxe2x80x9d, Lawrence Livermore National Laboratory, UCRL-53344, November 1982.
According to this embodiment of the invention preparation of small waveguide laser sources that produce high output power and that exhibit low pumping threshold levels and high slope efficiency for generation of laser emission compared to the prior art can be achieved, for example.
A further preferred embodiment of carrying out ion exchange, in the manner described above or otherwise, the inventors have discovered that carrying out the process in the absence of ambient atmosphere is advantageous due to the lessening of surface etching due to reactions of the glass surface with water vapor in the ambient air. According to this embodiment of the invention, processing conditions that allow successful fabrication of waveguide structures in phosphate glass substrates by ion exchange technology without the substrate exhibiting signs of chemical attack that influence waveguide integrity or quality are described.
This embodiment of the invention relates to waveguide lasers that are capable of high power operation and that exhibit low pumping threshold levels and high slope efficiency. These waveguides are prepared in rare earth doped phosphate laser glasses using ion exchange technology. Two basic technologies are commonly employed. In the first, ions of one type are exchanged for ions of another type within selected regions of a glass surface. The ions are selected such that the refractive index within the desired waveguide is higher than in the surrounding substrate glass. Here, the ion source is from a molten bath, often described as a salt batch, into which the glass is submerged.
Common salt baths contain alkali salts such as potassium nitrate, sodium nitrate, and/or other salts such as silver nitrate. In addition to a requirement of higher index within the waveguide, a second criteria in salt bath identity selection is to select ions that are highly mobile within the glass structure and offer melting points consistent with acceptable diffusion rates and melting points of the employed salts.
In a second technique, the ions to be placed into the glass are first applied as a solid to one or more glass surfaces. Electrodes are then placed onto the glass surfaces (in some cases the ion source can also act as an electrode, for example in the case of silver ion exchange) and an electric field through the glass then assists in driving the ion exchange process.
Additionally, both types of ion-exchange techniques can be conducted to provide the desired waveguides.
Glasses used as substrates for the lasers of this invention are of a sodium-aluminum-phosphate formulation such as the compositions listed in, for example, U.S. Pat. No. 5,334,559. A simple example of waveguide fabrication to produce a waveguide laser in phosphate glass is disclosed in U.S. Pat. No. 5,491,708.
Phosphate glasses are typically characterized by inferior chemical durability, and are particularly susceptible to attack by acidic solutions and by direct water attack. This can be demonstrated by a review of chemical resistance tests on common commercial phosphate glasses, see for example the Schott Optical Glass Catalog. Water, although not strongly acidic in nature, attacks the phosphorous sites within the glass structure leading to the formation of a phosphoric acid film on glass surfaces which are then responsible for acid attack of the remaining glass.
Phosphate glasses also have inferior resistance to alkali. Examples of this phenomena are demonstrated in the Laser Glass Brochure from Schott. All of the phosphate laser glasses in the brochure have inferior alkali resistance when compared to the silicate laser glasses listed in the same brochure. Any OR existing in a salt bath can combine with the alkali ions present, such as K+ and/or Na+, and form an alkaline solution that will attack the surface of a phosphate glass submerged in the salt bath for ion exchange.
Schott Glass Technologies (SGT) and the National Institute of Standards and Technology (NIST) have engaged in a collaborative research effort to develop advanced guided wave laser sources through the fabrication of small compact laser waveguides in rare earth doped phosphate laser glasses. These waveguides were prepared within the glasses by ion exchange technologies, including replacement of sodium ions within the substrate glass by either potassium ions or silver ions from the surrounding environment. The ion exchange locations were first defined by preparation of a mask that defined the waveguide regions, typically narrow stripes on the glass surface of width less than 50 xcexcm.
Although the phosphate glasses developed for this work are designed to exhibit good chemical durability and have been characterized to offer superior chemical durability than conventional state-of-the-art phosphate optical glasses, the need for further improvement was identified. Particularly, it had been observed that resultant waveguides were of poor optical quality. In particular, glass surfaces were observed to be stained with a white appearance to the eye. In addition, sections of the waveguides were found to exhibit at various times the following defects: a) narrowing of the waveguide width at unpredictable locations, b) widening of the waveguide at unpredictable locations (independent of other defects), and c) depressed regions relative to the surrounding, unexchanged, glass substrate.
Although not intending to be bound by this theory, it was believed that a dominant mechanism for chemical attack of the phosphate glasses during ion exchange was by residual hydroxyl ions within the salt bath or trapped within the glass/electrode interface during field assisted ion exchange. The mechanism was as follows: OH groups from the surrounding environment enter the glass, breaking the basic phosphorous/oxygen back bone responsible for the glass structure, i.e.:
Pxe2x80x94Oxe2x80x94Pxe2x80x94Oxe2x80x94Pxe2x80x94Oxe2x80x94P+OHxe2x86x92 greater than Pxe2x80x94Oxe2x80x94Pxe2x80x94OH HOxe2x80x94Pxe2x80x94Oxe2x80x94P.
In moderate cases of attack, glass durability is further degraded by the loss of backbone structure. In severe cases of attack, phosphorous could even be expected to leave the glass and migrate to the surface, presenting the white appearance observed in many of the defects within prepared waveguides.
The dominant source of water in molten salt bath ion exchange processes was thought to be residual hydroxyl within the starting salts since these chemicals are inherently hydroscopic. Ways to deal with control of this water were considered: addition of agents to the salt bath to xe2x80x9ctie upxe2x80x9d, including powdered aluminum, silicic acid, anhydrous aluminum oxide, diatomaceous earth, free P2O5, halogen containing chemicals that would react with OH to form volatile halide gases, etc.; and, bubbling of a dry gas through the salt bath to achieve direct water removal (dehydration).
A second source of water is the ambient air within the ion exchange furnace. Here, effective corrective actions discussed were: purging the oven environment (preferably prior to, but also during salt bath insertion) while at elevated temperatures with a dry gas and isolation of the salt bath from the environment (since the refractory within most furnaces acts as a sponge for atmospheric water during periods of inactivity that is later released upon application of high temperature to the furnace).
A third source of OH groups can be actual adsorbed water on the surface of the glass part to be treated. Phosphate glasses are known to adsorb water onto their surfaces from the atmosphere. Diffusion coefficients for water into phosphate glasses have been evaluated at several orders of magnitude higher than silicate glasses for the same measurement temperature. Such surface absorbed water can provide hydroxyl that can further re-react with glass surfaces during later processing, for example during field assisted ion exchange.
A number of measures were studied to understand, and control or eliminate the waveguide defects:
1) The current aluminum ion exchange crucible would be modified to allow possible bubbling of dry, and possible reactive, gases through the melt prior to insertion of glass substrates. The role of reactive gases was stressed during the meeting. Normal xe2x80x9cdryxe2x80x9d gases might quickly reach chemical equilibrium with the moisture in the salt bath, preventing the gas bubble to further dry the melt. A reactive gas within the melt (in particular, the use of chlorine) would react with water diffusing from the melt into the bubble, effectively maintaining the chemical gradient between the melt and bubble and allowing additional drying action to take place.
2) The ion exchange oven would be equipped with gas purge lines to pressurize the oven for purposes of moisture removal. Care would be taken to purge the oven before insertion of the salt bath since the oven refractory were considered as a major moisture source when heated.
3) Alternative crucible materials would be considered. In particular, Schott had reason to believe from trials to ion exchange phosphate glasses for purposes of chemical tempering that fused silica crucibles might offer improved performance in terms of chemical attack of treated glasses.
4) The addition of drying or other agents that could xe2x80x9ctie upxe2x80x9d hydroxyl groups in the melt was also discussed but no clear implementation plan was apparent at the close of the review meeting.
5) A fixture would be designed to allow placement and or removal of a preheated glass plate into a preheated and presumably dry salt bath without opening the oven to the surrounding ambient air within the ion exchange laboratory. This step would provide an opportunity to keep the glass/salt bath system isolated from room air during the entire course of the ion exchange process.
6) Since molten salt baths are expected to accumulate hydroxyl, or other contamination with time from the surrounding environment and treated glass plates, salt baths would be changed more frequently, in particular upon first visual evidence of a defect problem.
The corrective steps described above make possible the preparation of waveguide laser sources without the occurrence of waveguide defects linked to chemical attack of the phosphate glass substrate. This should enable the easier commercialization of these waveguide sources. Without this invention, reproducible performance levels in devices fabricated in a large scale manufacturing environment would be expected to exhibit erratic performance and/or low yield with associated high manufacturing costs due to unpredictable chemical attack from residual hydroxyl groups with associated defect creation that impacted device performance.
Additional embodiments of this invention are directed to optical devices having multiple waveguides on a substrate. The processes described above, while also applicable to preparing singular waveguides and lasers based thereon, are additionally applicable in preparing substrates having multiple waveguides and lasers based thereon.
A substrate containing multiple waveguides can be provided by photolithographic techniques and ion-exchange methods discussed above. Both involve applying a mask over the substrate to define the channels which will constitute the waveguide. By applying a mask with multiple channels over the substrate, a substrate with multiple waveguides therein can be formed. Additionally, multiple waveguides having differing refractive indices, and thus capable of providing lasing at differing wavelengths when incorporated in a laser device, can be formed by applying a mask which defines channels of differing width. The resulting waveguides formed by photolithography or ion-exchange techniques will have differing widths and thus differing refractive indices and can, thus, be used in a laser device to provide a laser capable of operation at differing wavelengths depending upon which waveguide is pumped. In other words, the waveguides form laser-resonator cavities with distinct resonance characteristics to provide a distinct lasing action at a selected wavelength when pumped. These waveguides can also be modified in any of the manners discussed above and below to vary their properties. The substrates used to provide the multiple waveguide devices are preferably based on the glasses discussed above.
Another embodiment of the invention is directed to modifying or tuning the wavelengths of a waveguide or waveguides in a substrate. This can be done by heating of the substrate which will alter the wavelengths of the waveguides therein. Where the substrate containing waveguide(s) is part of a laser device, it was expected that the heating thereof would increase the wavelength of the laser due to expansion of the diffraction grating periodicity. What the inventors have discovered, however, is that for substrates containing solid state waveguides provided as channels in the substrate, as discussed above, heating has a fine tuning effect on altering the wavelength of the waveguide. Thus, for example, while semiconductor DFB lasers are increased in wavelength upon heating, the increase of wavelength upon heating of laser devices with waveguides according to this invention is significantly lower as a function of the temperature, eg., the increase of wavelength as a function of temperature is roughly 15 times lower than that for semiconductor GFB lasers. The inventors have discovered that while heating expands the glass increasing the wavelength, the extent of increase is offset by the temperature effecting a decrease of the refractive index with temperature of the glass forming the waveguide(s). The theory behind this and experiments supporting it are described in the Journal of Non-Crystalline solids (JNCS) article xe2x80x9cRigorous Scalar Modeling of Er and Er/Yb-doped Waveguide Lasersxe2x80x9d (Veasey, Gary, Amin), particularly at page 14 and in FIG. 14. According to the invention, therefore, the temperature control requirements for maintaining a stable wavelength are relaxed with the waveguides according to the invention, i.e., a variance in the temperature will not have as significant effect on the tuning, allowing finer tuning thereof.
In another aspect of the invention it may be of advantage to fabricate active waveguide devices, either having one or multiple waveguides, that employ regions of active (for example, rare earth doped) and passive (for example lanthanum doped) glass sections. This is preferably done by preparing the active and passive sections separately and bonding them together. Examples of the sections to be bonded together include the single or multiple waveguide devices discussed above. There are a number of techniques possible to complete the bonding operation, including the use of UV or thermally curable epoxy adhesives available on the open market. There are other techniques that do not require adhesive materials, including a process wherein the parts to be bound are joined by a high temperature fusion process.
According to the invention, such bonded parts can be formed in the following manner. Glass blocks to be joined are first cut to a size suitable to yield one or more finished devices. The surfaces to be joined are then ground and polished. A high quality polish is not a requirement but is an advantage. However, simple grinding with 600 grit emery paper is adequate. Next, the samples are thoroughly cleaned using techniques well known in the optical industry. The surfaces to be joined are then placed in direct contact, forming a multiple laminate structure. A fusion of the glass pieces is then conducted by heating the entire assembly to a temperature above, generally about 100xc2x0 C. above, the glass transformation point, Tg, for a period of time to effect fusion thereof, preferably between 0.5 and 2 hours. The fused parts are then cooled, for example, to about 30xc2x0 C. above Tg, held there to allow thermal uniformity, for example for 2-4 hours, followed by a slow cooling ramp drop, e.g., of 30xc2x0 C. to 50xc2x0 C., to room temperature. The joined fused parts can then be cut, ground, and polished as needed. The waveguide can be provided in the fused blocks before the fusion takes place, in which case they must be aligned to be in communication, or the waveguide can be provided in the fused block after fusion thereof.
For a glass such as NIST- 1T discussed above, the glass transformation point, Tg, as measured by dilatometry is 474xc2x0 C. Consequently, a good fusion temperature is 575xc2x0 C. to 590xc2x0 C., with controlled cooling onset point of about 500xc2x0 C. Note that the softening point of the NIST-1T glass is in the range of 560xc2x0 C. to 565xc2x0 C. It is thus equivalent to say that the fusion is performed at a temperature above the softening point of the glass.
One disadvantage of the fusion process is that the joined parts need to be closely matched in thermal expansion and in Tg, e.g., in the range of xc2x130xc2x0 C. for Tg and in the range of xc2x11 to xc2x110xc3x9710xe2x88x927/K in thermal expansion. Higher mismatches in thermal expansion lead to high levels of residual stress at the glass/glass boundary points due to unequal thermal contraction during the cooling of the joined assembly to room temperature. This criteria is readily satisfied by substituting lanthanum for the active rare earth ions in the NIST-1T glass to prepare the passive material. In this way, the changes in these properties are minimized without altering the sodium-aluminum-phosphate base glass composition. If necessary, the base composition can also be modified to bring the Tg and thermal expansion values closer, but this can be done without seriously altering the ion exchange properties so that waveguide fabrication in both the passive and active portions of the prepared assembly can be conducted with minimal adjustment to accommodate different diffusion properties that could alter ion exchange behavior.
Waveguides prepared in accordance with any of the above descriptions, having multiple or single waveguides of the same or differing wavelengths, are useful in preparing lasers by providing the waveguide with a grating pattern. Examples of methods for producing lasers from waveguides of the type discussed above are provided in xe2x80x9cArrays of Distributed-Bragg-Reflector Waveguide lasers at 1536 nm in YB/ER-co-doped Phosphate Glassxe2x80x9d (Veasey, Funk, Sanford, Hayden); xe2x80x9c170 mW eb at 1540 nm from an Erbium/Ytterbium Co-doped Glass Waveguide: Laserxe2x80x9d (Funk, Veasey, Peters, Sanford, Hayden); and xe2x80x9cIon-exchanged Er+/YB3+ Glass Waveguide Lasers in Silicate Glassesxe2x80x9d (Peters, Veasey, Funk, Sanford, Houde-Walter, Hayden), which are incorporated herein by reference. These references also discuss methods generally applicable to production of waveguides and those teachings are additionally incorporated by reference herein. In general, lasers are fabricated from the waveguides by providing a reflecting element at both ends of the waveguide. The reflecting elements can be those known in the art. Included as embodiments are waveguides having optically polished ends provided with mirrors on both ends. An additional preferred embodiment, is providing the waveguide with a diffraction grating on one end of the waveguide. In a preferred embodiment, the grating is provided by etching onto the glass substrate containing the waveguide(s). One preferred type of grating is a DBR grating as known in the art. Such grating are advantageous because they provide a narrow reflection line and thus provide a laser with a narrower wavelength.
One embodiment provides making the photoresist grating by evaporation of a chromium coating with the specimen inclined 60 degrees to the normal of the evaporation source beam. This results in a hard metal coating on the tops of the photoresist bars that constitute the grating and thus allows a more selective sputter etch of the grating and the use of higher DC bias voltages for the etching process.
One embodiment provides a DBR grating exposure in standard photoresist using an optical phase mask. Additionally, providing a SiO2 buffer layer coating by sputtering or CVD deposition will reduce surface scattering losses. While it is difficult to get standard photoresist to propertly adhere to phosphate glasses, application of sputter deposited SiO2 on the surface of the phosphate glass, e.g., 1-2 nm, will greatly improve adhesion of the photoresist during processing.
One embodiment provides for actively monitoring the diffraction efficiency of the photoresist grating mask during photoresist developing. This optimizes the contrast of the photoresist grating and thereby provides for enhanced etch selectivity.
One embodiment provides etching of the grating by Ar-ion sputtering in a standard Reactive-Ion-Etching system. No CFC (chlorinated fluorocarbon) emissions, as with reactive-ion-etching of silica glass, are observed.
One embodiment provides many gratings that are exposed in photoresist fabricated on each single glass chip on a wafer in a single exposure using a silica plate that has multiple pitch phase masks printed on it.
One embodiment provides a nearly sinusoidal grating without excess loss due to improved isotropic plasma etching using only Argon gas at low pressure.
One embodiment provides an isotropic etching in an argon ion plasma leaving a smooth surface resulting in lower grating scattering losses.
One embodiment provides each grating with accurate line width control due to phase mask exposure of photoresist and control of differential linewidth using phase masks with more than one period on a single substrate.
One embodiment provides a laser wavelength that is selected by printing a single pitch grating over an array of optical waveguides with each waveguide in the array having a different refractive index.
One embodiment provides a laser wavelength that is selected by fabricating a single pitch grating on an array of identical optical waveguides where each waveguide crosses the grating at varying angles.
One embodiment provides a mode field shape and size that is optimized for laser operation using a filed assisted ion-exchange process where the electric field is varied as a function of time.
In one embodiment, the invention can provide a monolithic single-frequency waveguide laser in a spectroscopically superior phosphate glass, for example, with an initial laser linewidth was 500 kHz.
The present invention provides predictable emission wavelength if effective index of the waveguide is known.
The present invention provides high power, e.g., up to 80 mW (previous state-of-the art for an Er-doped DBR laser was xcx9c2 mw by Technical University of Denmark Microelectronic Centrct. Technology was sputter deposited silica doped with Er).
The present invention demonstrates a Yb/Er-doped phosphate glass laser using a DBR surface relief grating. The previous best device like this utilized an external fiber grating and produced power of 2.2 mW at 70 mW coupled pump power by A. Yeniay, et al., Lehigh University and Lucent, Apr. 29, 1997 in IEEE Photonics Technology Letters. This Yeniay demonstration did not operate as a single frequency laser.
The present invention provides high slope efficiency of 26 percent (previous state-of-the-art for similar devices was about 11 percent by Yeniay, et al., Lehigh University and Lucent).
The following provides some specific embodiments of features discussed above, however, the invention is in no way limited thereto.
The present invention provides a process for forming waveguides onto (or into) the surface of a glass substrate. In one embodiment, photo-lithographic techniques define waveguides by changing the index of refraction of waveguide channels formed into the surface of the substrate. In one such embodiment, a glass wafer, approximately 10 cm by 10 cm by 1 mm is cut from a slab of IOG-1 laser glass available from Schott Glass Technologies, Inc., of Duryea, Pa., USA. The surfaces of interest, including a xe2x80x9ctopxe2x80x9d major surface (where xe2x80x9ctopxe2x80x9d refers to an orientation and not necessarily to the placing of the device as used in operation) are polished to optical smoothness. The glass wafer will have a plurality of segments according to one embodiment of the present invention.
In one embodiment, the top surface of the wafer will be processed to form a large number (e.g., in one embodiment, one hundred devices) of individual segments (each forming an optical system) which will be diced apart and individually packaged. For example, in one embodiment, each segment is a portion of the wafer diced to about 1.5 cm by 0.5 cm.
Another embodiment is directed to segment having forty laser waveguides organized in eight sets. The segments may be processed, for example, according to one of the methods described in xe2x80x9cArrays of Distributed-Bragg-Reflector Waveguide Lasers at 1536 nm in YB/ER-co-doped Phosphate Glassxe2x80x9d (Veasey, Funk, Sanford, Hayden); xe2x80x9c170 mW eb at 1540 nm from an Erbium/Ytterbium Co-doped Glass Waveguide Laserxe2x80x9d (Funk, Veasey, Peters, Sanford, Hayden); and/or xe2x80x9cIon-exchanged Er3+/YB3+ Glass Waveguide Lasers in Silicate Glassesxe2x80x9d (Peters, Veasey, Funk, Sanford, Houde-Walter, Hayden), to form a plurality of sets (e.g., in one embodiment, each set has five waveguides; and in another embodiment, each set id used such that one waveguide is used, and the other four provide redundancy in case one or more do not function properly). In this embodiment, each set is overlaid with a diffraction Bragg reflector (DBR) which forms one mirror of a laser, and each DBR is fabricated to a different spacing designed to resonate at a different output wavelength. In one embodiment, only eight of the forty waveguides are used for eight respective lasers; the others are provided for redundancy. Thus, the DBR for one set is designed such that all five waveguides of that set will lase at the same wavelength, and any one of these waveguides can be used as the laser for the desired wavelength of that set. However, each of the DBRs are designed for a different output wavelength. Thus the segment is designed to provide eight lasing waveguides each outputting light at one of eight predetermined wavelengths that are tuned by the eight DBRs. In one embodiment, and input mirror (e.g., a multi-layer dielectric mirror) is deposited on an end face of segment opposite the DBRs. In other embodiments, an external mirror is placed against that face to provide the feedback function desired for lasing and the pump-light-launching function. The input mirror is designed to transmit as much light as possible at the pump wavelength (in one embodiment, 0.98 micrometers), while reflecting as much light as possible at the output wavelength (in one embodiment, a selected wavelength near 1.54 micrometers as tuned by the corresponding to the DBR). In one embodiment, the segment is used in a communications system that uses dense wavelength-division multiplexing (DWDM), wherein, for example, forty different wavelengths are each modulated to carry a different channel of information, and then all forty channels are passed on a single optic fiber. In one such embodiment, each channel""s wavelength differs from the next channel""s wavelength by 0.8 nanometers. Thus, for example, a segment could be designed to output laser light at wavelengths of 1.5360, 1.5368, 1.5376, 1.5384, 1.5400, 1.5408 and 1.5416 micrometers. Thus, a forty-channel system only needs five such different part numbers (i.e., unique parts designs), rather than forty different part numbers in conventional approaches.
In another embodiment a segment is used as part of a multi-wavelength laser source. In this embodiment, a pump light source (for example, a semiconductor laser diode lasing at about 0.98 micrometers; this source can even be very xe2x80x9cnoisy,xe2x80x9d emitting a range of frequencies from perhaps as low as 0.96 and as high as 1.00 micrometers, and still provide a suitable source for laser emitting at e.g., 1.5360 micrometers) is coupled through a suitable launch-end mirror by butting against the end of a selected waveguide of a set of waveguides. In one such embodiment, eight pump laser diodes are used. At the other end of the segment, eight optic fibers are held by an alignment block, such that each respective fiber is optically coupled to the emitting end of the top or first waveguide of the eight respective sets. If any one of the first waveguides is non-functional, the alignment block can be moved down to couple to the next waveguide of the eight sets.
In another embodiment, a segment is used as part of another multi-wavelength laser source with the output fibers connected to the middle waveguide of each set of waveguides. A different pumping mechanism can be used wherein each pump is coupled through a lens (which can be a conventional convex-type lens or a diffraction or holographic lens) that helps concentrate the light from the pumps into the waveguides.
In another embodiment, a segment is used as part of a single-wavelength laser source. This embodiment provides a single output wavelength selected from the eight available. This embodiment also provides an optical fiber for coupling light from a pump laser diode into the desired waveguide. It is to be understood that any of the above pump-coupling methods and apparatus could be used in the other discussed embodiments.
In another embodiment, a segment having a laser comprising a waveguide, DBR mirror and optional input mirror is provided. The single laser has an external launch mirror. Other embodiments include redundant waveguides all operating at a single wavelength, other waveguides each having a DBR tuned to a unique wavelength, or both, all integrated on a single segment.
In another embodiment, a packaged device that includes a segment according to one of the other embodiments of the present invention is provided. The device includes a hermetic package, and is pumped, in this embodiment, by a fiber, and coupled to an output fiber. While this embodiment includes a laser as described above, it is otherwise a passive device. Such a device takes a pump light input, which can be noisy as described above, and outputs a xe2x80x9ccleanxe2x80x9d laser output light at a different frequency, wherein the output is stable and has a very narrow wavelength spectrum.
In another embodiment an integrated device that includes a segment according to one of the other embodiments of the present invention is provided. The device includes active components such as a pump laser diode, a photodetector (e.g., a light-sensing diode), a thermistor and electrical connections between and to these components. Some embodiments also include a temperature maintaining device (such as a resistor or thermoelectric device to heat and/or cool device) in order to maintain a constant temperature. Some embodiments also include an output modulator to encode information on the output light. Some such embodiments also include an output combiner to combine the modulated light into a single fiber.
In another embodiment, a laser using direct (butt) coupling of pump laser diode to a segment as described in other embodiments is described.
In another embodiment, a top laser using lensed coupling of a pump laser diode to a segment according to one of the embodiments of the present invention is provided.
Lasers incorporating waveguides according to the invention have potentially narrower line widths than current state-of-the-art semiconductor lasers primarily because of the long upper state lifetime of the Er laser manifold in phosphate glass. The Schallow-Townes relationship gives an ultimate limit for solid state lasers of less than 1 Hz compared to approximately 10-100 kHz for diode lasers. The lasers of this invention have been demonstrated to have line widths of less than or equal to 500 kHz, much better than the typical 1 MHZ laser line widths for standard WDM lasers sources used today.
The relative intensity noise (RIN) of solid state waveguide lasers is a distinct advantage over semiconductor lasers for analog fiber telecommunications. This is especially true as analog modulation frequencies increase to multi-GHz transmission rates as is expected to become common place in the near future. The only excess contribution above the shot noise photon statistics is the relaxation oscillation peak which occurs around 500 kHz. Beyond 500 kHz, the RIN drops rapidly in waveguide lasers. This, combined with high glass saturation power, means very low noise lasers can be achieved. In contrast, semiconductor lasers typically have relaxation oscillations between 3 and 10 GHz, which is a potentially important frequency range for optical analog communications.
There are advantages of putting the DBR grating within a passive section of glass attached to the active glass containing the laser waveguide. This is because when the pumplight reaches the DBR grating, it is not reflected but is instead coupled out of the waveguide at an acute angle, thus the pump light intensity gets depleted very rapidly as it traverses in the DBR grating. If the DBR grating were prepared in erbium doped glass, the resultant population of excited erbium ions within the active glass inside the DBR grating leads to an increase in the spontaneous emission noise. And also introduces a loss within the laser cavity which reduces the efficiency.
Another embodiment of this invention is the fabrication of a single-frequency 1.32-1.4 um laser in Nd-doped phosphate glass fused to La-doped glass, as shown in FIG. 2. In this invention, a waveguide is fabricated in the Nd-La fused substrate using the ion exchange process described above. A DBR grating is written in the La section of the substrate. It is necessary to write the DBR grating in the passive section of the substrate since Nd acts as a three level laser system near 1.3 um. Such a design minimizes loss and noise as described above and thus increases the efficiency of the laser. The period of the grating is fabricated to be such that it reflects a single wavelength in the range of 1.32-1.4 um thus produces a single frequency laser in this range. Many water absorption peaks occur in this region of the optical spectrum, therefore such a laser could be used as a high power laser source for the detection of low level water concentration in high purity gases.
Finally, the glass waveguide lasers discussed here will not suffer at all from wavelength drift as in semiconductor lasers. As a semiconductor laser ages, the high optical intensity in the laser cavity causes a change in the index of refraction of the semiconductor material, and consequently a change in the optical characteristics of the laser cavity that results in the wavelength of the laser drifting over time. At some point, the change in the material is so severe that a longitudinal mode hop occurs that causes a large wavelength shift since longitudinal modes in semiconductor lasers have large frequency spacing. When such a event occurs in a WDM network, the effect is usually catastrophic and may bring down the whole network for a short time while the problem source is isolated. This effect is not well studied in phosphate glasses but our intuition is that the effect will not be large. Since the longitudinal modes are also very close (within WDM channel tolerances) mode hopping will not be as severe an effect if aging does in the phosphate glass.
Thus, the devices described here are capable of replacing current semiconductor based laser sources for application in telecommunications and data transmission applications within the 1.5 um telecommunications band. Output powers exceed that required (20 mW) when pumped with inexpensive 980 nm class pump lasers available on the market today.
The devices described can also provide small, compact laser sources for localized optical networks operating at either 1.5 um or near 1.0 um through the use of erbium and neodymium rare earths respectively. In particular, the inventors forsee the need for inexpensive, compact, efficient sources at 1.0 um for use in localized networks within individual aircraft, automobiles, seacraft, etc. Here, the short distance for optical transmission allows the use of conventional optical fiber (that is optimized at 1.3 um and 1.5 um but suffers from dispersion issues at 1.0 um) without accumulated distortion from dispersion effects, in combination with the additional advantage of employing less costly, and easier to operate and maintain than detectors at 1.3 um or 1.5 um, detectors that are sensitive at 1.0 um.
A wide variety of rare earth species and combinations of species can be supported in the glass formulations of this invention. Combined with the laser structures of this invention, a number of laser wavelengths can be made available as coherent light sources for diagnostic and spectroscopic analysis completed with optical techniques.
The entire disclosure of all applications, patents and publications, cited above and below, as well as of U.S. provisional application No. 60/117,477 filed Jan. 27, 1999, and U.S. provisional application No. 60/162,458 filed Oct. 29, 1999, from which this application claims priority are hereby incorporated by reference in their entirety.